It’s often claimed that the maximum hourly carbohydrate intake that can be tolerated during exercise without causing stomach upset is ~90 grams per hour, but there's been a growing trend for athletes to report consuming more than this supposed 90g ceiling.
We take a look at whether more than 90g per hour can be tolerated and whether it's beneficial to your performance...
How athletes consume 90g of carbs per hour
Athletes generally achieve this 90g of carbs per hour by ingesting glucose and fructose in a 2:1 ratio.
Glucose - a carbohydrate absorbed and used rapidly - uses a different transporter protein (SGLT1) to pass into the bloodstream to fructose (a slower absorbed carb). Whereas the SGLT1 transporters don’t become saturated until a consumption of roughly 60 grams per hour, the fructose GLUT5 transporters’ capacity caps out at ~30g/h (or so it’s believed, but more on this later).
The 2:1 concept made headlines in 2004 and raised many athletes' perceived ‘ceiling’ of carbohydrate absorption from the previous theoretical limit of ~60g/hr (when just taking glucose into account).
In the wake of this finding, it wasn’t long until the performance benefits were also reported in the scientific literature and the upper tolerable limit of exercise carb ingestion was reset to 90g per hour.
But fast-forward from the early 2000s to now and reports of elite athletes consuming significantly more than 90 grams per hour (and tolerating it!) are fairly common.
What does the evidence say about consuming more than 90 of carbs per hour?
Take marathon legend Eliud Kipchoge. He’s reported to have been utilising carbohydrate intakes of greater than 100g/h in both his Breaking2 attempts back in 2017 and 2019.
In our work with elite athletes we've certainly observed similarly high levels of intake in cyclists and triathletes but you should not just take our word for it.
For example, Norwegian triathlete Gustav Iden, the winner of the 2019 IRONMAN 70.3 World Champs and 2020 Challenge Daytona, suggested that the introduction of significantly higher carbohydrate intakes to his racing strategy has been one of the biggest contributors to his success:
I have done quite a bit of work on improving my ability to absorb high levels of carbohydrate on hard efforts.
During Daytona, I had around 110g of carbohydrate per hour and that’s including the 30 minutes of the swim and start of the bike where I didn’t drink anything, so at times it was a higher rate than this.
Furthermore, a paper published in 2020 inadvertently tested the validity of such high carb doses, though its primary aim was to investigate the effects of high carbohydrate intake on exercise-induced muscle damage. What was remarkable about this paper was not just the high amount of carbohydrate given to the participants – 60, 90 and 120 grams per hour - but also the sport used to test it: running.
Classically, runners aren’t thought to be able to tolerate the same degree of carb intake as cyclists (there’s a reason it’s called the runners’ trots!). This is largely thought to be a result of the high-impact and repetitive mechanics of running which can cause greater stress and damage to the intestinal lining. 60-70 g/h is typically considered the standard recommendation for ultrarunners and other endurance athletes.
However, Aitor Viribay Morales (the paper’s lead author and a performance nutritionist with World Tour cycling team, Astana) found that a 120g/h carb intake could be tolerated successfully in some, if not all, ultra runners. A total of seven participants completed a 42-km race with no stomach problems when consuming the 120g/h dose (delivered as a 30g energy gel every 15 minutes), as well as a further seven who were taking the 90g/h dose (a gel every 20 mins).
On the whole, it’s an intriguing piece of research (and an impressive gastrointestinal feat from the participants) which sparks many further questions, not least, how do athletes ingest such a high carbohydrate intake and not suffer from gut distress?
How do athletes consume more than 90g of carbs per hour?
In short, it's widely believed that your gut has the capability to adapt to increases in the amount of carbohydrate ingested over time, helping you become more efficient at absorbing it.
Gut training may well explain how up to 90 grams of carbohydrate per hour is achievable, but it doesn’t directly address the additional 30 grams. Remember, as far as the science says to this point, your gut cannot readily absorb more than 90 grams of carb per hour because its glucose and fructose transporters should be saturated by this point.
Despite the lack of evidence at this stage, some possible theories are being discussed:
- Through adaptations driven by training on higher carbohydrate intakes, we’re able to learn to absorb more glucose. Certainly from conversations we’ve had with some of the elite and professional teams we work with, it’s plausible that the highly-trained can consume more than 60 grams of glucose per hour with seemingly no adverse effects. Whether this is solely through practice, or due to some innate characteristics (or a combination of both) is unclear, but it’s definitely happening in the field on a regular basis.
- The second explanation is that fructose actually plays a bigger role than we think and the current limit of ~30g/hr isn’t the concrete ceiling for absorption that has been suggested to this point. Viribay in particular has expressed his interest in looking closer at the fructose transporter (GLUT5).
Do you need more than 90g of carbs per hour?
So far, there appears to be increasingly solid evidence underpinning the idea that more than 90 grams of carb per hour can be tolerated by some athletes, some of the time, and indications that it may be beneficial to performance (at the elite level at least).
When it comes to offering a plausible understanding for how athletes are tolerating these larger amounts of fuel, researchers are still scratching their heads to an extent. But given the impact such a finding could have on future sports performance, you can bet there are people working hard to figure it out.
What we can all take from this is that consumption of really high levels of carbs must be built up slowly to give your gut time to adapt. It’s also important to recognise that just because carb intakes of greater than 90 grams per hour can sometimes be tolerated, it doesn’t necessarily mean that they should be.
Like your hydration plan, your fueling strategy should be optimised to you as an athlete and cater to your needs and physiological characteristics. A lower consumption (30-60 grams per hour) may be the most appropriate approach for some, whereas others thrive on higher intakes.